Only use PO4A on desktop file and manpages on non-Windows builds

Fixes the MXE-with-PO4A build
Merge pull request #246 from OdyX/dev-fix-mxe-build

Only use PO4A on desktop file and manpages on non-Windows builds
Add russian language support
Merge pull request #248 from OdyX/dev-add-russian

Add russian basic support
Fix tab-vs-space in CMake configuration file
Reorder desktop/CMakeLists.txt for more platform-specific clarity
Drop redundant custom_target for po4a
Updated data submodule
Updated data submodule
Russian translation of interface (#249)
Fixed missing "\n" in ru.po
Under MacOSX, generate an Info.plist
Generate an icns icon on Mac OSX
On Mac OS X, write colobot.ini and savegames to Application Support directories
Abstract the definition of DataPath to CSystemUtils
On MacOSX, define the DataPath as being the Resources path in the bundle
On MacOSX, don't cache the Data, Sound and Music paths

As users can potentially move the bundle around, it doesn't make sense to cache these

(It would probably make sense to drop this caching on all architectures, but it's only on MacOSX where it hurts most)
Abstract the definition of LangPath to CSystemUtils
On MacOSX, define the LangPath as being i18n in the Resources path of the bundle
Add MacOSX bundle-specific installation paths